Many processes within the chemical industry suffer from heat and mass limitations. These limitations result in increased processing times, reduced plant capacity, excessive product inventory and may even result in reduced yields. A relatively new design strategy, named Process Intensification (PI), has been developed in order that these process limitations can be minimized. The resulting processes are often smaller, more efficient, safer, lighter and all so importantly cheaper than the original process. The strategy can deliver these improvements by utilizing unit operations in which heat and mass transfer rates are matched to those required by the process. This ideology can more easily be achieved by converting batch processes to continuous processes such that small elements of fluid can be continuously exposed to the required hydrodynamic and thermal environments.
It has been known for a long time that fatty acid alkyl esters can be produced from either the base catalyzed transesterification or acid catalyzed esterification of a triglyceride with an alcohol. It is interesting that the process has been extensively used during periods of war as a source of the co-product, glycerol, which was used for the production of explosives. However, it is the long chained ester which is now receiving the attention as it is a potential fuel which can offset a significant fraction of the current demand of diesel fuel. Many natural plant sources of triglycerides exist, including but not limited to soy, canola, palm and cotton seed oils. Animal derived oils, such as but not limited to, beef tallow and fish oil have also been found to be acceptable sources. Commonly, methanol is used in the production producing a Fatty Acid Methyl Ester (FAME) which is often referred to as biodiesel. If other alcohols are used the alkyl ending of the chain (and name) takes that of the alcohol used. In the reaction the triglyceride molecule is fragmented into three smaller FAME molecules and one glycerol molecule. The process overcomes many of the problems associated with attempts to utilize the triglycerides directly, which predominately result from difficult atomization, due to the high viscosity and poor vaporization and combustion characteristics. The reaction reduces the viscosity of the oil by close to an order of magnitude (depending on feedstock) and results in a fuel with similar physical characteristics and energy density as conventional number two fuel. It has also been extensively shown that significant emissions are reduced through the combustion of neat biodiesel or biodiesel blends. This is attributed to the partially oxygenated nature of the molecule which vastly reduces the probability of any products leaving the combustion zone in any other form other than the products of complete combustion.
Biodiesel has been traditionally produced using batch reactor technology. Typically the oil is preheated and fed into the reaction vessel. The alcohol and catalyst are mixed and also added to the reaction vessel. The vessel is often jacketed to minimize heat loss and maintain reaction temperature. The process is often conducted at temperatures around 150° F. A reflux condenser is often used to prevent pressurization of the system and capture any alcohol vapors. An internal impeller is often used to provide shear to extend the contact between the two phases. If the same vessel is to be used as a decantation vessel to separate the heavier glycerin phase from the biodiesel phase this stirring is sometimes reduced or stopped towards the end of the reaction to prevent further breakup and size reduction of the glycerin droplets. Sometimes an external device such as centrifuge is used to perform this separation.
Biodiesel production based on batch technology suffers from many drawbacks and limitations. Firstly it is well known that the kinetic rate of the transesterification reaction is considerable at the temperatures typically used in the batch reactor. However, mass transfer is so limited by the poor hydrodynamic environment created by a single impeller that the process exhibits a large induction period. This induction period is primarily attributable to poor mixing between the immiscible oil and alcohol phases. Even after the passing of this induction period the time conversion profile does not approach that dictated by the kinetics of the process. A batch process typically allows two hours for the reaction to proceed to equilibrium; however, if mass transfer limitation were removed the process can be completed within minutes. This time saving allows vastly different reactive systems to be designed which allow rapid rates of reaction to be achieved as well as resulting in high final conversions.
In the case where the same reaction vessel is used as the decantation vessel then the time for settling will be very long. Typically reaction vessel has a height to diameter ratio greater than 1. Thus a glycerin droplet at the very top of the vessel has a considerable distance to fall before it arrives at the biodiesel glycerin interface. Assuming negligible convective or thermal currents exists and based on an average drop size then Stoke's Law can be used to estimate the average drop velocity and hence the time for settling. If a secondary continuous separation device is utilized, such as a centrifuge, than overall turn around time of the process may be reduced. This is generally only true if a relatively large device is used such that it can process the entire contents of the reactor faster than the contents would naturally settle. Thus, for this technique to be desirable large continuous devices must be utilized. Due to the financial as well as maintenance issues attached with such a technique this is not deemed to be a desirable solution.
Some processes utilize two batch reactors in series in a continuous manner. Utilizing a batch reactor in this manner is often referred to as a Continuously Stirred Tank Reactor (CSTR). In the process a CSTR is continuously fed with a preheated mixture of oil, alcohol and catalyst. Simultaneously product is continuously removed from the same vessel at the same rate as the feed. The size of the vessel is calculated to give sufficient average residence time in the vessel for the process to proceed to the desired point. The product of the first CSTR is directed to a separation device where the biodiesel and glycerin are separated. The partially reacted biodiesel mixture is then fed into a second CSTR, along with more catalyst and alcohol, where the second stage occurs. Again product is continuously removed and the glycerin separated. The second vessel is sized such that the conversion of the outlet stream is that required from the process.
The above process can be broadly described as a continuous process but still suffers from the same mass transfer limitation inherent in the batch system. It is well known that the outlet composition of a CSTR is the same as the average composition within the vessel. This effect reduces the driving force of the process resulting in slower kinetics. These two factors result in systems with extremely large holdup times which greatly complicates startup and shut down and serious process and safety issues must be addressed. Also, the conversion in the second CSTR can never achieve equilibrium as fresh feed is always being mixed into the system. Thus such a system can never achieve really high conversion and this may impose significant future implications if currently legislation regarding biodiesel were to change.
There exists a number of U.S. patents directed to biodiesel fuels including U.S. Pat. No. 6,015,440 issued to Noureddini. Triglycerides are reacted in a liquid phase reaction with methanol and a homogeneous basic catalyst. The reaction yields a spatially separated two phase result with an upper located non-polar phase consisting principally of non-polar methyl esters and a lower located phase consisting principally of glycerol and residual methyl esters. The glycerol phase is passed through a strong cationic ion exchanger to remove anions, resulting in a neutral product which is flashed to remove methanol and which is reacted with isobutylene in the presence of a strong acid catalyst to produce glycerol ethers. The glycerol ethers are then added back to the upper located methyl ethyl ester phase to provide a biodiesel fuel. Noureddini teaches an apparatus that includes a centrifuge; however, Noureddini does not teach an intensified process for the production of biofuel.
U.S. patent application No. 2003/0175182 applied for by Teall et al. describes a process for the small scale production of biodiesel. The process can be described by the steps of mixing, reaction, separation, distillation and filtration. The alkaline catalyst is dissolved in a low order alcohol and is co-fed with the triglyceride to the reaction vessel. An external stream is removed from the reaction vessel and passed through an array of centrifuges where glycerin is separated from the biodiesel. The biodiesel is returned to the reaction vessel. The glycerin stream enters a distillation column where the excessive methanol is recovered and recirculated to the reaction vessel. Teall teaches of a process to produce biofuels; however, Teall does not teach of a modular process capable of high throughput in which each unit operation is optimized through the use of process intensification.
U.S. patent application No. 2006/0021277 applied for by Petersen et al. describes a continuous process for the production of biodiesel. The process consists of two Continuously Stirred Tank Reactors (CSTRs) each with an external settling tank, arranged in series. The first CSTR is continuously fed with preheated triglyceride, catalyst and alcohol. Partial reaction occurs within the vessel. At the same rate as the feed is added to the CSTR a product stream is removed and is directed to a gravity separation tank. Here the glycerin gravity separates from the biodiesel to form a dense layer at the bottom of the tank. Through careful choice of exit pipes arrangements the tank has a self controlling mechanism to maintain the interface at the desired set point. The biodiesel over flow is directed to the second CSTR where the process is repeated. The two volumes of the tank and overall residence time in the process is large, hence, Petersen does to teach of a process exhibiting optimized kinetics resulting in a small process fluid inventory.
U.S. Pat. No. 6,364,917 issued to Matsumura et al. teaches a process for producing a fuel from plant oil. Matsumura teaches a method of heating the oil, mixing the oil with water and/or ozone, and agitating the mixture of oil and water and/or dissipating the ozone. The oil is then separated from the water. Matsumura further teaches equipment for producing the fuel including an agitator tank and an optional, separate centrifuge. Matsumura does not teach a process with minimized footprint.
U.S. Pat. No. 6,712,867 to Boocock discloses a process for the esterification of a triglyceride. The process comprises forming a single phase solution of a triglyceride in an alcohol selected from methanol and ethanol. The solution further comprises a co-solvent in an amount to effect formation the single phase and a base catalyst for the esterification reaction. After a period of time, ester is recovered from the solution. The esters may be used as biofuel or biodiesel. The process may be optionally operated in a batch or continuous process. However, Boocock does address the issues relating to the optimization of heat and mass transfer issues throughout the process.
U.S. Pat. No. 5,908,946 to Stern et al. discloses a process for the production of linear monocarboxylic acid esters with 6 to 26 carbon atoms. Vegetable oils or animal oils are reacted with monoalcohols having a low molecular weight, for example 1 to 5 carbon atoms, in the presence of a catalyst that is selected from among zinc oxide, mixtures of zinc oxide and aluminum oxide, and the zinc aluminates that correspond to the formula: ZnAl2O4, x ZnO, y Al2O3 (with x and y each being in the range of 0-2) and having more particularly a spinel type structure of an ester that can be used as a fuel or combustible and a pure glycerine. The process may optionally be operated in a continuous process that includes several autoclaves and decanters. Thus, Stern does not teach of an intensified process for the continuous production of biodiesel fuel.